The hazards presented in the event of a fire by large vessels containing pressurized fluids, particularly flammable fluids such as railcars for the conveyance of liquefied petroleum gases, are well known. The pressure within such a vessel when subject to external heat such as that due to an external fire, will rise, whilst the temperature of walls of the vessels, particularly in areas not in contact with the liquid contents of the vessels, may rise to a level at which their structural strength is sufficiently reduced to result in catastrophic failure to sustain the increased pressure within the vessel. Since such fires commonly arise following an accident such as a derailment, the orientation of the vessel when required to sustain such an abnormal position is not predictable.
The desirability of fitting a vessel with an efficient pressure relief valve is widely recognized and the performance requirements for such a valve are onerous and must be capable of providing an adequate rate of fluid flow under wide range of emergency conditions. It must retain a reclosure capability such as to restrain unnecessary continuation of venting of the contents of the vessel when its internal pressure has fallen to an acceptable level even after exposure to extreme temperatures. According to the circumstances and the orientation of the vessel during venting, the valve may be required to pass gas, liquid or a mixture of gas and liquid. The rate of volume discharge of gas will be much higher than that for liquid or a liquid/gas mixtures. The valve must be gas and fluid tight under non-emergency conditions as well.
Conventional pressure relief valves for this type of application have usually been of the poppet type, and rating for a desired vapour at a discharge rate. The configuration of a poppet valve is such as inherently to limit the valve opening which can be achieved with a valve body of a given size, and the difference between opening and closing pressures is fixed by the design of the valve.
The requirement for safety relief devices to resist a predetermined set pressure and to fully flow at only a slight increase in pressure, requires that the springs be sufficiently rigid to attain the set pressure and as well have a low enough spring rate to allow the valve to fully open during a necessity for full flow. Traditionally this requirement demands relatively large springs, which are aligned with the stem of the valve creating excessive profiles and obstructions in the fluid flow through the valve.
Some attempts have been made to solve this problem in particular U.S. Pat. No. 5,411,056 to Alberto Solaroli entitled Pressure Relief Valve and U.S. Pat. No. 4,856,547 to Alberto Solaroli again entitled Pressure Relief Valve has attempted to provide the necessary valve opening forces without having to resort to extremely large springs . I refer particularly to U.S. Pat. No. 4,856,547 in column 3, lines 15 to 25 where the inventor describes the use of a longitudinal slot 44 shown in FIG. 3 having a distal end within which is retained a sliding boss 46 formed with a groove 48 which engages an end loop 50 of a tension spring 52. As described in this patent, once the internal pressure is great enough to overcome the set force or set pressure, crank arm 42 pivots from the vertical position thereby allowing sliding boss 46 to move down a longitudinal slot 44 in the crank arms distal end, thereby attempting to reduce the spring bias on said valve as the valve opens from a closed position to an open position. This particular arrangement is subject to galling and friction forces on sliding boss 46 in longitudinal slot 44 which may be unpredictable and subject to the lubrication and materials employed.
The present invention allows for a small highly rated spring to supply the set resisting force with any further deflection of the spring providing a constant force or other force profiles as the application requires during opening.
Unlike conventional safety relief valves used in the storage and transportation of bulk fluids using the stem and seat arrangement, the present valve swings away from the flow channel allowing virtually unrestricted flow directly exiting the area that is under excess pressure. The design is such that the circular valve opens away from the seat and from the plane of mounting. Subsequently, extremely high flow rates can be achieved using this design.
Using the presently invented mechanism, the valve can for example operate as though the springs where providing constant force, opening from the set or closed position to a fully open position with little or no increase in restraining force. As the valve is opened by a pressure increase in the containment vessel or line, the component of restraining force can for example become smaller and remains less than the fluids static and dynamic force.
It is the objective of the invention to provide a pressure relief device, that is not constrained by the size of the springs, which directly act as the resisting force against normal operating pressures and events of over pressuring.